When heating a load in the form of food by means of a microwave oven, there are a number of aspects which have to be considered. Most of these aspects are well-known to those skilled in the art and include, for instance, the desire to obtain uniform heating of the food at the same time as a maximum amount of available microwave power is absorbed in the food with a view to achieving a satisfactory degree of efficiency.
In order to achieve uniform heating of the load, microwave stirrers and/or a rotating plate; on which the load is to be placed, have earlier been used.
In order to provide efficient coupling of microwaves to the cavity in a microwave oven, it has previously been suggested that a microwave source having a controllable frequency might be used. U.S. patent specification U.S. Pat. No. 4,196,332, for example, discloses such a microwave oven that works according to a predetermined pattern. First a frequency scan within a predetermined range is carried out, during which reflections from the cavity are detected, and the frequencies which give the lowest reflection are stored in a memory. Then the cavity of the microwave oven is fed with microwaves of a predetermined frequency which gives a low degree of reflection. The same specification also suggests jumping between a plurality of more or less optimal frequencies.